1. Field of the Invention
The present invention relates to a device and method for setting up label switched paths (LSPs) in a label switching network and, more specifically, to a device and method for setting up LSPs through the use of a distance vector type routing protocol in an IP (Internet Protocol) network that includes a plurality of label switching routers (LSRs).
2. Description of the Related Art
FIG. 1A shows a conventional IP packet transfer in an IP network. In FIG. 1A, when IP packet transfer is made by routers #1, #2, and #3, each router refers to the destination address set in each IP packet and performs software-based transfer processing in the third layer of the basic reference model of OSI (Open System Interconnection).
In contrast, in multi-protocol label switching (MPLS), which is now being standardized, a fixed-length label of 20 bits is allocated for an IP packet flow specified by an FEC (Forwarding Equivalence Class).
An FEC specifies a group of IP packets, such as the flow of IP packets for an individual application or the flow of IP packets having the same destination network. Each router performs fixed-length-label-based switching in the 2.5-th layer by hardware. In such a label switching network, a path that can make packet transfer using a label is referred to as an LSP.
FIG. 1B shows an IP packet transfer by routers for MPLS. In FIG. 1B, routers #1, #2 and #3 correspond to an ingress LSR, transit LSR, and egress LSR, respectively.
Here, the ingress LSR is one that attaches a label to a packet with no label at the entrance of an LSP. The egress LSR is one that removes the label from the labeled packet at the exit of the LSP. The transit LSR is located between the ingress and egress LSRs and transfers the labeled packet.
For example, the router #1 attaches a label #a to an incoming IP packet 1 with no label and then transfers the resulting IP packet 2 to the router #2. The router #2 changes the label of the received IP packet 2 to #b and then transfers the resulting IP packet 3 to the router #3 . The router #3 removes the label from the received IP packet 3 and then transfers the resulting packet 4 with no label to the destination network. Thus, IP packets can be transferred at high speed by performing switching using fixed-length labels.
When combined with a routing protocol, the MPLS can recognize the network topology to set up LSPs automatically. The routers can benefit from high-speed routing only by making the setting of the MPLS function effective. Thus, this function is expected to be in increasing demand and become the future standard function of routers.
Routing protocols that are combined with MPLS include open shortest path first (OSPF) protocols, routing information protocols (RIP), etc.
The OSPF, in such a network configuration as shown in FIG. 1C by way of example, creates such a shortest path tree as shown in FIG. 1D to compute the shortest route to a destination network. In FIG. 1C, A, B, C, D, E and F represent routers and a, b, c, d, e, f and g represent router-to-router networks. The numerical value associated with each network indicates the transfer cost for that network.
The shortest path tree of FIG. 1D, provided in the router A, hold the shortest route for which the transfer cost is minimum when an IP packet is transferred from the router A to another router. For example, the shortest route from router A to router C is the route from A through B to C and the transfer cost is 20. This value is determined by adding together the cost for the network a between the routers A and B and the cost for the network c between the routers B and C. In the same manner, the shortest path tree is provided for other routers.
In the combination of OSPF and MPLS, each router can refer to the shortest path tree to set up a single LSP for networks that the same router accommodates. Thus, the LSPs can be prevented from increasing in number.
On the other hand, RIP is a distance vector type routing protocol, which is a routing protocol that, in order to cause a frame (IP packet) to arrive at its destination network, manages only the next router (next hop) and the distance (the number of hops) to the destination network.
The RIP determines next hops by creating such a routing table as shown in FIG. 1E in the network of FIG. 1C. The routing table of FIG. 1E is provided in the router A and manages the next hop and cost for each destination network. Here, the number of hops is used as the cost instead of the cost value in FIG. 1C. The RIP is now the most used protocol owing to ease of management and installation and is expected to continue being used in the future as well.
However, the aforementioned conventional routing protocols have the following problems:
The OSPF protocol is very difficult to manage at its operation time because of complexity of its specifications. For this reason, the OSPF is little used at present. Also, since installation itself is difficult, the combination with the MPLS involves much complexity.
On the other hand, in the combination of a distance vector type routing protocol, such as RIP, with the MPLS, it is impossible for the ingress LSR to identify a router that accommodates the destination network. For this reason, even with networks under the same egress LSR, an LSP is to be set up and a different label is to be allocated for each network address. Thus, when the number of networks accommodated by a router is increased, a large number of labels would have to be used.
With MPLS, in order to swap the label in the transit LSR, one might suggest using a label lookup table by which the label (outgoing label) of the outgoing IP packet to the next hop is retrieved according to the label (incoming label) of the incoming IP packet.
However, when a large number of labels is used as described above, the number of entries in the label lookup table increases, increasing the time required for retrieval and lowering the packet transfer capability.